2,6-Dihydroxynaphthalene is a compound useful as a starting material for the production of synthectic resins, synthetic fibers, pharmaceuticals, agrichemicals, dyes, etc.
A classical method for the production of 2,6-dihydroxynaphthalene consists of subjecting 2-naphthol-6-sulfonic acid to alkali fusion with potassium hydroxide. This method, however, has the problems of low yield and difficult separation from the tar content. Furthermore, the end compound 2,6-dihydroynaphthalene is prone to crystallization and the inorganic salt used in a large amount as a reactant tends to be incorporated in the crystal of 2,6-dihydroxynaphthaelen, making it difficult to be obtained with high purity.
It is also known that 2,6-dihydroxynaphthalene can be produced by first oxidizing 2,6-diisopropylnaphthalene with molecular oxygen in the presence of a base to form diisopropylnaphthalene dihydroperoxide, then acid-decomposing this peroxide with an acidic catalyst such as sulfuric acid. A problem with this method is that when 2,6-diisopropylnaphthalene is oxidized with molecular oxygen in the presence of a base, not only is the end compound 2,6-diisopropylnaphthalene dihydroperoxide (hereinafter sometimes abbreviated as DHP) obtained but also various by-products are formed in large quantities. Among these by-products are carbinols such as 2-(2-hydroxy-2-propyl-6-(2-hydroperoxy-2-propyl)naphthalene (hereinafter sometimes abbreviated as HHP), 2,6-bis(2-hydroxy-2-propyl)naphthalene (hereinafter sometimes abbreviated as DCA) and 2-isopropyl-6-(2-hydroxy-2propyl)naphthalene (hereinafter sometimes abbreviated as MCA), and monohydroperoxides such as 2-isopropyl-6-(2-hydroperoxy-2-propyl)naphthalene (hereinafter sometimes abbreviated as MHP). Furthermore, when the reaction product obtained by oxidation of 2,6-diisopropylnaphthalene and which contains not only DHP but also various by-products is subjected to acid decomposition in the presence of an acid catalyst such as sulfuric acid, the end compound dihydroxynaphthalene is obtained together with various products of acid decomposition reaction such as isopropylnaphthol.
As described above, if 2,6-diisopropylnaphthalene is oxidized to DHP by reaction with molecular oxygen in the presence of a base and if the resulting DHP is subjected to acid decomposition with an acidic catalyst such as sulfuric acid, the reaction mixture obtained will contain not only the desired dihydroxynaphthalene but also large amounts of various by-products of the reaction.
It is known that 2,6-dihydroxynaphthalene can be purified by recrystallization using solvents such as alcohol, ether, acetone, acetic acid, benzene and water. However, none of these solvents are completely satisfactory for recrystallization purposes because their ability to dissolve 2,6-dihydroxynaphthalene is either too high or too low to attain good results.
Under these circumstances, other purification methods have been reviewed. In one method, cumeme or some other suitable material is added to the reaction product of acid decomposition or, in some instances, the concentrate obtained by removing the solvent and other materials that have been employed in the reaction of acid decomposition, and the resulting precipitate of crude 2,6-dihydroxynaphthalene is sublimated to obtain pure 2,6-dihydroxynaphthalene. Another method comprises using an aqueous alcohol or hydrous ketone as a solvent for recrystallization. Use of activated carbon for decolorizing purposes has also been reviewed. However, these methods have suffered the problem that satisfactory values of purity cannot be attained if an attempt is made to increase the recovery rate and that the decoloring effect is not high enough for practical purposes.